A display is an electronic component or subsystem used to convert electrical signals into visual imagery suitable for direct interpretation by a human observer. In the field of display devices those employing the gaseous glow discharge phenomenon are used in many applications. The terms gas discharge display and plasma display and plasma panel are each used to describe displays of this type. Because of a uniquely favorable combination of electrical and optical properties, the neon gas discharge is the one invariably used for plasma display purposes. Although all electric discharges result in the generation of visible light, only Ne-based gas mixtures are capable of producing visible radiation having brightness and efficiency that are suitable for display applications; these displays are always orange in color. Since other rare gas mixtures do not have the luminous efficiency of neon, to achieve an acceptable visible brightness level so that other gases can be used the gas discharge must be driven at more than ten times the drive current or duty cycle, a factor that severely restricts their use for practical application.
Rare gas mixtures are used for other purposes in the display art, however. In U.S. Pat. No. 4,013,912 of D. C. Hinson, a gas discharge mixture comprised essentially of argon and xenon in approximately equal proportions is disclosed. Although such a mixture produces no visible emission of practical use, reactions in the gas discharge result in the formation of excited molecular states of argon and xenon which decay producing radiation in the vacuum ultraviolet region. The ultraviolet radiation excites color phosphors which are incorporated in the display and which subsequently produce visible emission after bombardment by the UV photons. Incorporation of the phosphor in display devices has proven to be difficult and costly, however, and problems with display resolution and phosphor lifetime are significant. For these reasons hybrid plasma displays of the gas discharge/phosphor type as disclosed in the Hinson patent are still in a developmental stage and offer little promise as a replacement for neon based displays for applications where the latter can be used.
Another approach employs a mercury-based argon mixture developed by O. Sahni and reported in a paper in the 1980 SID International Symposium Digest of Technical Papers, April 1980. Using this mixture blue emission was obtained from an AC plasma panel with a luminous efficiency and brightness comparable to that of Ne-based displays. However, the strong temperature dependence of the mercury vapor pressure results in electrical and optical characteristics that are very sensitive to temperature, and mercury cannot be used in certain applications for safety reasons, factors that practically preclude the use of the mercury-argon mixture for displays.
In articles published in Applied Physics Letters 40, 223 (1982) and in the IEEE Transactions on Electron Devices ED-30, 439 (1983) W. L. Nighan and C. M. Ferrar reported on the feasibility of utilizing visible fluorescence from excimer molecules for plasma displays. The term excimer is used widely in the laser art in reference to excited molecular species which are not stable in the ground electronic state. Excimer molecules are semistable and do not exist in the absence of the discharge. Rather, excimer molecules are generated by the action of the discharge on the initial gas mixture constituents and then produce visible radiation upon dissociation.
The prior art also contains a commonly owned U.S. patent application, Ser. No. 625,199, filed June 28, 1984, a continuation of U.S. Ser. No. 322,098, filed Nov. 16, 1981 of W. L. Nighan, et al, entitled "Optical Display With Excimer Fluorescence".
Disclosed in the application is a plasma display using visible excimer molecule fluorescence of a heteronuclear excimer such as Xe.sub.2 Cl, XeO and XeF. Using an AC plasma panel, monochrome emission in the blue/green region of the spectrum was obtained using gas mixtures in which either Xe.sub.2 Cl or XeO was produced. However, XeF is unique among the rare gas monohalide excimer molecules in that it alone exhibits both ultraviolet and visible transitions, the existence of the former interfering severely with the generation of visible radiation. Additionally, XeF is the only rare gas monohalide excimer that is destroyed by collisions with both the rare gas atom from which it is made, xenon, and by the source of fluorine from which it is produced, F.sub.2, factors that severely restrict conditions under which XeF can be formed.
Although the visible transition of XeF has been demonstrated in the laser art, the pumping energy densities used for lasers are several orders of magnitude larger than those permissible for display applications and the gas pressures used are several times larger than atmospheric, an intolerable condition for display technology. For these reasons means to utilize visible XeF excimer molecule fluorescence are unknown in the prior display art.
Although plasma displays deriving their visible emission from the fluorescence of neon gas are far superior to those using any other prior art gas mixture, neon-based displays can only produce an orange color, which is not acceptable for some applications, and their brightness is insufficient under conditions of high ambient light. Since changing the gas mixture is, in principle, the easiest way to change the color of a gas discharge display, efforts to develop a practical alternative to the Ne-based plasma display mixture have been proceeding on many fronts for well over ten years, but no suitable alternative has been found. An article by L. F. Weber appearing in the book Flat Panel Displays (L. E. Tannas, editor), Van Nostrand and Reinhold, New York 1985 provides a comprehensive review of the state of plasma information display science and technology up to the present time.